The biosynthesis and actions of prostaglandins in insect immune tissues

I report on the biosynthesis and actions of prostaglandins (PGs) in insect immune tissues. PGs, oxygenated metabolites of arachidonic acid, are important mediators of insect cellular defense reactions to infections. While PGs and PG biosynthesis have been recorded in a few insect tissues, there remains a large lacuna in our understanding of PG biosynthesis in insect tissues. Chapters 2 and 3 add new data on PG biosynthesis by fat body (a major immune tissue) from larvae of true armyworm, Pseudaletia unipuncta, and the tenebrionid beetle, Zophobas atratus. Fat bodies from both species produced four PGs, PGA2, PGD2, PGE 2 and PGF2α. PGA2 and PGE2 were the predominant products in true armyworm fat body whereas PGA2 and PGF2α were predominant products in beetle fat body. Indomethacin and naproxin effectively inhibited PG biosynthesis in both fat body preparations. Unlike the tenebrionid beetle and other invertebrates (as well as mammalian systems) studied to date, the true armyworm fat body did not require the presence of the usual exogenous co-factors (reduced glutathione and hemoglobin) for optimal PG biosynthesis. I infer that, as seen in most animals, these two insect species are competent to biosynthesize PGs, although we can not yet outline the general principles of PG biosynthesis in insects. In mammalian systems, PGs influence cells through their interactions with specific PG receptor sites. Here, I also report the first biochemical characterization of an insect PG receptor in hemocytes prepared from tobacco hornworms, Manduca sexta. The binding of (3H)PGE 2 was saturable and specific. Mathematical analysis indicate the presence of a single population of PGE2 binding sites with a KD approximately 35 pmol/mg protein and Bmax approximately 7.5 fmol/mg protein. The influence of purine nucleotides on binding suggests these are functional G protein coupled binding sites. I infer from these results that the mechanism of PG action in insect cellular immune physiology involves specific G protein couple PG receptors. This research marks a meaningful advance in our understanding of PG biosynthesis and actions in insect immunity